Riders on the Blue and Orange Metro lines encountered more than a little delay this morning. But more critical than the delay was the confusion and panic that accompanied a mid-tunnel evacuation among the commuters. The story has spread like wildfire and already begun to appear in national media. Yet no one is still entirely sure what happened. What we do know is, largely, not especially encouraging.

Furthermore, several hundred Metro riders had a front-row seat to the accident and uncertainty. Luckily we don't have to imagine just how strange and unfortunate their morning was. Video has already emerged showing the train suspension and evacuation.

Here's what Metro rider Fabricio Torres described as "the first moments of panic" between L'Enfant and Smithsonian:

Reports and rumors immediately began circulating. Why was a Metro train stopped near L'Enfant Plaza and what was the cause? Was there fire or smoke or even a fireball?

A wheel popped off a train, some said, citing an anonymous Metro source. WMATA quickly dispelled this notion. People were told it an "obstruction" blocked the Metro tunnel. This confusion reigned for a couple hours in the morning as Blue and Orange line service was suspended until shortly after noon, when it began single-tracking. But about 300 people had to be evacuated in complete darkness throughout these hours. Luckily, no one was harmed and all was resolved.

Rider Torres also captured and tweeted the following video of the pitch-black evacuation through Metro tunnels. He later joked of mole men and sparks and tweeted: "I've been trapped on the metro for two hours thanks to a train fire. On my train." Today, Mr. Torres, was clearly not your day:

WMATA is still investigating the specific causes, I understand, but released the initial diagnosis: "The preliminary investigation has found that the track obstruction was part of a rail car called a 'friction ring' that became dislodged and fell to the track bed from a passing Orange Line train at approximately 9:40 a.m. The friction ring made contact with the electrified third rail, resulting in a smoke condition until power was de-energized shortly after the incident occurred."

All right. A "friction ring." Photographer Laura Elizabeth Pohl captured a photo of the ring on the Vienna/Franconia-Springfield-bound Metro tracks, initially mistaking it for a wheel. To say riders saw "fire" or a "fireball" seems dramatic; I imagine there was quite a flare but we're probably talking about third-rail arcing, which riders frequently spot as trains depart.

I quickly found myself wondering precisely what this device represented.

Search WMATA.com for the phrase and nothing turns up. The Internet turns up a few images and notes about what this component might look like. It's a simple ring of metal, from what I can tell. Search for "friction brake," however, and several pages turn up. Perhaps "friction ring" is the right, precise term, but I wonder if WMATA used it to distance itself from the idea that it was a brake problem. The initial Metro announcement above makes no mention of brakes, for instance, although the transit agency isn't hiding the fact in interviews.

Friction brakes have posed the occasional problem in the past. WMATA described "friction brake problems" in its 2000- and 3000-series cars back in 2005, I notice. These brakes are also responsible for that "unpleasant squeal noise," as the Volpe Center noted in an old WMATA report, throughout our transit system.

WMATA contains information about their friction braking system here in the 2008 Running Maintenance and Servicing Manual for the Metrorail 6000-series cars. It's right here in chapter 4: "Friction Brake Equipment." Certain safety features are designed into the friction brakes:

Through this configuration, a single failure of one of the FB.ECUs or TCUs will lead to a reduction of braking effort of not more than 25 percent (of the total braking effort on a per married pair basis).

...The loss of one FB.ECU or the detection of failure from DIA_WHITE_LIGHT GROUP (as defined in the Event Code List) will also cause a white light condition. Due to the system configuration of four FB.ECUs per married pair, a loss of a single FB.ECU results in a loss of only 25 percent of braking effort on a per married pair basis. A loss of two FB.ECUs on a car results in an automatic emergency brake application by opening the normally open relays in the emergency loop.

How's that for jargon? What the manual describes is good for the train's speeding but what caused the delays today is the part itself. It fell from the train and hit the third rail.

But what I consider most are the commuters. People were late for work, they trudged through total darkness, they lost time and likely had to walk and pay more to get to their jobs. No one pays their Metro fare expecting that, despite the lesser complications WMATA offers customers on a regular basis. Metro riders experienced a healthy dose of fear this morning, and I imagine and hope WMATA will conduct a full investigation into what happened with the friction ring. A tiny piece of metal has a profound ability to disrupt what's an extraordinarily delicate system, we've learned today. I'm glad everyone is safe.